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SI r10 ch48

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This file is licensed to Abdual Hadi Nema (ahaddi58@yahoo.com) License Date: 6/1/2010 Related Commercial Resources CHAPTER 48 ULTRALOW-TEMPERATURE REFRIGERATION Autocascade Systems Custom-Designed and Field-Erected Systems Single-Refrigerant Systems Cascade Systems 48.1 48.2 48.2 48.3 Licensed for single user © 2010 ASHRAE, Inc U LTRALOW-TEMPERATURE refrigeration is defined here as refrigeration in the temperature range of –50 to –100°C What is considered low temperature for an application depends on the temperature range for that specific application Low temperatures for air conditioning are around 0°C; for industrial refrigeration, –35 to –50°C; and for cryogenics, approaching K Applications such as freeze-drying, as well as the pharmaceutical, chemical, and petroleum industries, use refrigeration in the low temperature range as designated in this chapter The –50 to –100°C temperature range is treated separately because design and construction considerations for systems that operate in this range differ from those encountered in industrial refrigeration and cryogenics, which bracket it Designers and builders of cryogenic facilities are rarely active in the low-temperature refrigeration field One major type of low-temperature system is the packaged type, which often serves applications such as environment chambers The other major category is custom-designed and field-erected systems Industrial refrigeration practitioners are the group most likely to be responsible for these systems, but they may deal with low-temperature systems only occasionally; the experience of a single organization does not accumulate rapidly The objective of this chapter is to bring together available experience for those whose work does not require daily contact with low-temperature systems The refrigeration cycles presented in this chapter may be used in both standard packaged and custom-designed systems Cascade systems are emphasized, both autocascade (typical of packaged units) and two-refrigerant cascade (found in custom-engineered low-temperature systems) AUTOCASCADE SYSTEMS An autocascade refrigeration system is a complete, self-contained refrigeration system in which multiple stages of cascade cooling effect occur simultaneously by means of vapor/liquid separation and adiabatic expansion of various refrigerants Physical and thermodynamic features, along with a series of counterflow heat exchangers and an appropriate mixture of refrigerants, allow the system to reach low temperature Autocascade refrigeration systems offer many benefits, such as a low compression ratio and relatively high volumetric efficiency However, system chemistry and heat exchangers are complex, refrigerant compositions are sensitive, and compressor displacement is large Operational Characteristics Components of an autocascade refrigeration system typically include a vapor compressor, an external air- or water-cooled condenser, a mixture of refrigerants with descending boiling points, and a series of insulated heat exchangers Figure is a schematic of a simple system illustrating a single stage of autocascade The preparation of this chapter is assigned to TC 10.4, Ultralow-Temperature Systems and Cryogenics Low-Temperature Materials 48.6 Insulation 48.9 Heat Transfer 48.9 Secondary Coolants 48.10 Fig Fig Simple Autocascade Refrigeration System In this system, two refrigerants with significantly different boiling points are compressed and circulated by one vapor compressor Assume that one refrigerant is R-23 (normal boiling point, –82°C) and the second refrigerant is R-404a (normal boiling point, –46.7°C) Assume that ambient temperature is 25°C and that the condenser is 100% efficient With properly sized components, this system should be able to achieve –60°C in the absorber while the compression ratio is maintained at 5.1 to As the refrigerant mixture is pumped through the main condenser and cooled to 25°C at the exit, compressor discharge pressure is maintained at 1524 kPa (gage) At this condition, virtually all R-404a is condensed at 35°C and then further chilled to subcooled liquid Although R-23 molecules are present in both liquid and vapor phases, the R-23 is primarily vapor because of the large difference in the boiling points of the two refrigerants A phase separator at the outlet of the condenser collects the liquid by gravitational effect, and the R-23-rich vapor is removed from the outlet of the phase separator to the heat exchanger At the bottom of the phase separator, an expansion device adiabatically expands the collected R-404a-rich liquid such that the outlet of the device produces a low temperature of –19°C at 220 kPa (gage) (Weng 1995) This cold stream is immediately sent back to the heat exchanger in a counterflow pattern to condense the R-23rich vapor to liquid at –18.5°C and 1524 kPa (gage) The R-23-rich liquid is then adiabatically expanded by a second expansion device to –60°C As it absorbs an appropriate amount of heat in the absorber, the R-23 mixes with the expanded R-404a and evaporates in the heat exchanger, providing a cold source for condensing R-23 on the high-pressure side of the heat exchanger Leaving the heat exchanger at superheated conditions, the vapor mixture then returns to the suction of the compressor for the next cycle 48.1 Copyright © 2010, ASHRAE Simple Autocascade Refrigeration System This file is licensed to Abdual Hadi Nema (ahaddi58@yahoo.com) License Date: 6/1/2010 48.2 2010 ASHRAE Handbook—Refrigeration (SI) Fig Four-Stage Autocascade System Licensed for single user © 2010 ASHRAE, Inc Fig Four-Stage Autocascade System As can be seen from this simple example, the autocascade effect derives from a short cycle of the refrigerant circuit within the system that performs only internal work to condense the lower boiling point refrigerant The concept of the single-stage cycle can be extended to multiple stages Figure shows the flow diagram of a four-stage system The condensation and subsequent expansion of one refrigerant provides the cooling necessary to condense the next refrigerant in the heat exchanger downstream This process continues until the last refrigerant with the lowest boiling point is expanded to achieve extremely low temperature The design process includes selection of (1) metal for piping and vessels and (2) insulating material and method of application The product to be refrigerated may actually pass through the evaporator, but in many cases a secondary coolant transfers heat from the final product to the evaporator Brines and antifreezes that perform satisfactorily at higher temperatures may not be suitable at low temperatures Compressors are subjected to unusual demands when operating at low temperatures, and, because they must be lubricated, oil selection and handling must be addressed Design Considerations Single-refrigerant systems are contrasted with the cascade system, which consists of two separate but thermally connected refrigerant circuits, each with a different refrigerant (Stoecker and Jones 1982) In the industrial refrigeration sector, the traditional refrigerants have been R-22 and ammonia (R-717) Because R-22 will ultimately be phased out, various hydrofluorocarbon (HFC) refrigerants and blends are proposed as replacements Two that might be considered are R-507 and R-404a Compressor Capacity As can be seen from Figures and 2, a significant amount of compressor work is used for internal evaporating and condensing of refrigerants The final gain of the system is therefore relatively small Compressor capacity must be enough to produce an appropriate amount of final refrigerating effect Heat Exchanger Sizing Because there is a significant amount of refrigerant vapor in each stage of the heat exchanger, the overall heat transfer coefficients on both the evaporating and condensing sides are rather small compared to those of pure components at phase-changing conditions Therefore, generous heat-transfer area should be provided for energy exchange between refrigerants on the high- and low-pressure sides Expansion Devices Each expansion device is sized to provide sufficient refrigerating effect for the adjacent downstream heat exchanger Compressor Lubrication General guidelines for lubrication of refrigeration systems should be adopted CUSTOM-DESIGNED AND FIELDERECTED SYSTEMS If refrigeration is to maintain a space at a low temperature to store a modest quantity of product in a chest or cabinet, the packaged lowtemperature system is probably the best choice Prefabricated walkin environmental chambers are also practical solutions when they can accommodate space needs When the required refrigeration capacity exceeds that of packaged systems, or when a fluid must be chilled, a custom-engineered system should be considered The refrigeration requirement may be to chill a certain flow rate of a given fluid from one temperature to another Part of the design process is to choose the type of system, which may be a multistage plant using a single refrigerant or a two-circuit cascade system using a high-pressure refrigerant for the low-temperature circuit The compressor(s) and condenser(s) must be selected, and the evaporator and interstage heat exchanger (in the case of the cascade system) must be either selected or custom-designed SINGLE-REFRIGERANT SYSTEMS Two-Stage Systems In systems where the evaporator operates below about –20°C, two-stage or compound systems are widely used These systems are explained in Chapter of this volume and in Chapter of the 2009 ASHRAE Handbook—Fundamentals Advantages of twostage compound systems that become particularly prominent when the evaporator operates at low temperature include • Improved energy efficiency because of removal of flash gas at the intermediate pressure and desuperheating of discharge gas from the low-stage compressor before it enters the high-stage compressor • Improved energy efficiency because two-stage compressors are more efficient operating against discharge-to-suction pressure ratios that are lower than for a single-stage compressor • Avoidance of high discharge temperatures typical of single-stage compression This is important in reciprocating compressors but of less concern with oil-injected screw compressors • Possibility of a lower flow rate of liquid refrigerant to the evaporator because the liquid is at the saturation temperature of the intermediate pressure rather than the condensing pressure, as is true of single-stage operation Refrigerant and Compressor Selection The compound, two-stage (or even three-stage) system is an obvious possibility for low-temperature applications However, at very low temperatures, limitations of the refrigerant itself appear: freezing point, pressure ratios required of the compressors, and This file is licensed to Abdual Hadi Nema (ahaddi58@yahoo.com) License Date: 6/1/2010 Ultralow-Temperature Refrigeration 48.3 Table Low-Temperature Characteristics of Several Refrigerants at Three Evaporating Temperatures Pressure Ratio with Two-Stage System Refrige Freezing rant Point Licensed for single user © 2010 ASHRAE, Inc R-22 –160°C R-507

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  • SI Table Of Contents

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  • Autocascade Systems

    • Operational Characteristics

    • Design Considerations

  • Custom-Designed and Field- Erected Systems

  • Single-Refrigerant Systems

    • Two-Stage Systems

    • Refrigerant and Compressor Selection

    • Special Multistage Systems

  • Cascade Systems

    • Refrigerants for Low-Temperature Circuit

    • Compressor Lubrication

    • Compressors

    • Choice of Metal for Piping and Vessels

  • Low-Temperature Materials

    • Metals

    • Thermoplastic Polymers

    • Thermosetting Plastics

    • Fiber Composites

    • Adhesives

  • Insulation

  • Heat Transfer

  • Secondary Coolants

  • References

  • Bibliography

  • Tables

    • Table 1 Low-Temperature Characteristics of SeveralRefrigerants at Three Evaporating Temperatures

    • Table 2 Properties of R-508b

    • Table 3 Theoretical Performance of Cascade System UsingR-13, R-503, R-23, or R-508b

    • Table 4 Theoretical Compressor Performance Data forTwo Different Evaporating Temperatures

    • Table 5 Several Mechanical Properties of Aluminum Alloys at –196°C

    • Table 6 Approximate Melting and Glass TransitionTemperatures for Common Polymers

    • Table 7 Tensile Properties of UnidirectionalFiber-Reinforced Composites

    • Table 8 Components of a Low-Temperature Refrigerated PipeInsulation System

    • Table 9 Overview of Some Secondary Coolants

    • Table 10 Refrigerant Properties of Some Low-TemperatureSecondary Coolants

  • Figures

    • Fig. 1 Simple Autocascade Refrigeration System

    • Fig. 2 Four-Stage Autocascade System

    • Fig. 3 Simple Cascade System

    • Fig. 4 Simple Cascade Pressure-Enthalpy Diagram

    • Fig. 5 Two-Stage Cascade System

    • Fig. 6 Three-Stage Cascade System

    • Fig. 7 Tensile Strength Versus Temperature ofSeveral Metals

    • Fig. 8 Tensile Elongation Versus Temperature ofSeveral Metals

    • Fig. 9 Shear Modulus Versus Normalized Temperature(T/Tg) for Thermoplastic Polymers

    • Fig. 10 Tensile Strength Versus Temperature ofPlastics and Polymer Matrix Laminates

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